Signs of a bad timing belt are, rough idling, engine misfires, hard starting, engine stalling, low oil pressure, squealing sounds or an engine does not start.
Other signs of a bad timing belt are ticking or clicking in the engine, engine smoking and broken pistons or valves.
A timing belt usually lasts between 100,000 to 150,000 miles so if your timing belt has not been replaced within 100,000 to 150,000 miles then you should get the timing chain replaced as it will eventually fail as they do become stretched.
The timing belt has to be installed correctly with the proper tension or it will not allow the engine to start or run properly.
If you're unsure on how to replace the timing belt then you should have a mechanic replace the timing belt because it needs to be installed properly.
Also sometimes it takes several hours to remove a lot of stuff on the vehicle to get to the timing belt and you need gaskets and seals to put the new timing belt on.